Many chemotherapeutic methods are now available to be used in the treatment of cancer. One of the most successful methods is the use of anti-mitotic agents which interfere with the assembly or disassembly of microtubules. Since microtubule assemble and disassemble is necessary for mitosis, inhibition of either the assembly or disassembly of microtubules interferes with cell proliferation. Thus, compounds that inhibit the assembly of microtubule are useful in treating diseases or conditions which are caused or exasperated by rapid or abnormal cell proliferation, such as cancer.
Several anti-mitotic agents have had considerable clinical success. For example, the following vinca alkaloids which inhibit microtubule assembly have proved clinically successful: Vincristine has been successfully used to treat hematological malignancies and non-small-cell lung carcinoma; Vinblastine has been successfully used to treat hematological malignancies, testicular carcinomas and non-small-cell lung carcinoma; and Vinorelbine has been successfully used to treat hematological malignancies, breast carcinomas and non-small-cell lung carcinoma. In addition, taxanes which inhibit microtubule disassemble have also proved to be clinically successful. For example, Paclitaxel has been successful in treating breast, ovarian and non-small-cell lung carcinomas; and Docetaxel has been successful in treating breast and non-small-cell lung carcinomas.
Despite these successes, available anti-mitotic agents are inadequate for a number of reasons. For example, paclitaxel, docetaxel and vincristine are associated with significant neuropathy which can limit their use in repeat courses of therapy. In addition, both the vinca alkaloids and taxanes are good substrates for the 170 kDa P-glycoprotein (Pgp) efflux pump found in most multi-drug resistant cells. This protein pumps a drug out of the tumor cells causing the tumor cells to become resistant to treatment. Once a patient's cancer has become multi-drug resistant, there is typically little that can be done to halt or retard further progression of the disease.
There is therefore still a need for new drugs which overcome one or more of the aforementioned shortcomings of drugs currently used in the treatment of cancer. Desirable properties of new anti-cancer drugs therefore include a good therapeutic index, efficacy against tumors that are currently untreatable or poorly treatable, efficacy against multi-drug resistant tumors and/or reduced side effects.